Gasoline engines are typically controlled to operate substantially around stoichiometric air-fuel ratio in order to balance fuel economy with emission conversion in exhaust emission control devices. However, gasoline engines controlled to operate with relatively lean combustion may experience significant fuel economy benefits compared to stoichiometric combustion. For example, air-fuel ratios between 20:1 to 28:1 may provide an optimal balance between fuel economy, combustion stability, and emissions, although lean operation produces increased NOx emissions compared to stoichiometric operation. The excess NOx may be converted by providing a selective catalyst reduction (SCR) system in the exhaust system.
However, the inventors herein have identified potential issues with the above approach. For example, SCR systems may experience maximum NOx conversion efficiency when exhaust oxygen concentration exceeds a lower limit, such as 8%. During lean operation with an air-fuel ratio of 25:1, for example, oxygen concentration may be lower than 8%, and hence NOx conversion efficiency may be degraded.
Thus, in one example, some of the above issues may be at least partly addressed by an engine method comprising, operating the engine with lean combustion, and when exhaust oxygen concentration is below a threshold, injecting air into an exhaust passage between a first emission control device and an SCR device.
In this way, the exhaust oxygen concentration may be adjusted via the introduction of secondary air into an exhaust passage between the upstream emission control device and the SCR device. The secondary air may be introduced based on feedback control to maintain exhaust oxygen concentration at a desired amount and/or maintain a temperature of the SCR device within a threshold range.
The SCR device is configured to convert NOx using an injected reductant, such as ammonia. By including the SCR system and injecting secondary air when oxygen concentration is low, efficient conversion of NOx during lean combustion may be provided, balancing fuel economy benefits from lean combustion with emissions control.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.